A semiconductor device fabricating process includes a step of forming a silicon nitride film on a process object such as a semiconductor wafer by using an ALD process.
The ALD process alternately supplies two (or more) different source gases one by one onto a process object under predetermined process conditions, so that chemical species is adsorbed to the process object at a single atomic layer level, and are deposited on the process object through surface reactions. For instance, first and second source gases are alternately supplied to a process object to flow along the surface thereof, thereby molecules contained in the first source gas adsorb to the surface, and molecules contained in the second source gas react with the adsorbed molecules originated from the first source gas to form a film of a thickness of a single molecule level. The above process steps are performed repeatedly, so that a high-quality dielectric film, specifically a film of a high dielectric constant (high-k), is formed on the process object.
JP2004-281853A discloses that, when forming a silicon nitride film by an ALD process by alternately supplying dichlorosilane (DCS: SiH2Cl2) gas and ammonia (NH3) gas, if the ammonia gas is supplied after it is activated to be converted into ammonia radicals, the silicon nitride film can be formed at a low process temperature in a range of 300° C. to 600° C.
However, a silicon nitride film deposited at a low process temperature by using an ALD process has a high wet-etch rate, and thus, the etching selectivity of the silicon nitride film over an oxide film is small. Moreover, a silicon nitride film deposited under a low process temperature has a low film stress, and cannot achieve desired stress strength.